1. Field of the Invention
The present invention relates to an operating circuit applied to a backlight, and more particularly, to an operating circuit applied to a light-emitting diode (LED) backlight and associated method.
2. Description of the Prior Art
Please refer to FIG. 1, which illustrates a prior art backlight module control system 100. As shown in FIG. 1, the backlight module control system 100 includes a LED string 110, a current control circuit 120 and a resistor Rext, where the LED string 110 includes a plurality of LEDs, and the current control circuit 120 includes an operational amplifier 122 and a transistor M1. In the operations of the backlight module control system 100, the current control circuit 120 uses the operational amplifier 122 to form a negative feedback mechanism so as to make a feedback voltage Vfb equal to a reference voltage Vref. Therefore, a stable current I_LED flows through the LED string 110, where the current value I_LED=(Vfb/Rext).
However, because of the semiconductor processing variation, there is an unavoidable mismatch present in an input stage of the operational amplifier 122. That is, the input stage of the operational amplifier 122 has an offset voltage ΔV. Therefore, in actual circuits, the current I_LED provided by the current control circuit 120 is influenced by the offset voltage ΔV of the operational amplifier 122, and the current I_LED of each current control circuit 120 may be different due to different offset voltage ΔV of the operational amplifier 122. When a plurality of LED strings 110 and current control circuit 120 form a backlight module, the currents I_LED of the LED strings 110 may be different, causing the luminance-uniformity of the backlight module to be degraded.
In addition, the backlight module control system 100 is generally operated under a high-voltage environment (i.e., a supply voltage Vo ranges from 30V to 60V), therefore, the current control circuit 120 is generally manufactured by a special high-voltage process rather than a low-voltage process.